1. Field of the Invention
The present invention relates to a hard coating material with improved sliding characteristics, particularly scuffing resistance as for instance used to cover the sliding surfaces of sliding members such as piston rings in internal combustion engines.
2. Description of the Related Art
In recent years, the environment in which piston rings are used has become increasingly harsh due to stricter exhaust emission regulations and higher engine output. To cope with this harsher environment, hard films utilizing physical vapor deposition such as TiN film, CrN film or a hard film with oxygen in a solid solution state in a crystal structure of CrN (Japanese Patent Laid-open No. 6-265023) and a hard film with carbon in a solid solution state in a crystal structure of CrN (Japanese Patent Laid-open No. 6-300130) have been proposed. Furthermore, a composite film of molybdenum nitride (Mo.sub.x N.sub.y) and chromium nitride (Cr.sub.x N.sub.y) having superior wear resistance (Japanese Patent Laid-open No. 8-199337) has been proposed. Additionally, a film of Cr--Mo--N--O alloy (Japanese Patent Laid-open No. 6-346077) has been proposed. The constituent proportions of the metallic elements of the film and the constituent proportions of the gas elements were merely presented in this official gazette without stipulating the crystal structure.
However, these proposed methods all have the drawback that scuffing resistance still deteriorates under harsh engine conditions.